The assembly of microtubules in crude supernatants has been studied using inhibitors. It was found sodium fluoride prevented the assembly of microtubules in crude supernatant prepared from brain, but not the assembly of purified brain tubulin. The effect of the particulate subfractions on assembly of microtubules was determined. The microsomal fraction contained an activity that inhibited assembly of microtubules in crude supernatant, but not the assembly of purified tubulin. Both inhibitions can be overcome with high levels of GTP. Work is in progress to determine the mechanisms of the inhibitions and the relevance to cell control of assembly-disassembly of microtubules. The kinetics of the reaction between colchicine and purified brain tubulin were investigated using a fluorometric assay for the formation of colchicine tubulin complex. The results support a multi-step binding reaction. Rate constants for the reaction were determined.